Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
The carbon nanotubes have interesting properties with regard to magnetoelectronic applications which utilize the spin properties of electrical charge carriers. Tsukagoshi et al “Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube” Nature 401: 572-574 (1999) discloses that carbon nanotubes transport the spin of conduction electrons without a spin flip-over process when conduction electrons are injected into a carbon nanotube. If conduction electrons with polarized spins are injected into a carbon nanotube, for example proceeding from a ferromagnetic conductor, then the spin orientation of the conduction electron is maintained in the carbon nanotube over a path length of approximately 250 nm.
U.S. Pat. No. 6,987,302 to Yingjian Chen and Xiaozhong Dang describes incorporating nanotubes into semiconductor devices, such as transistors, by attaching magnetic nanoparticles to the nanotubes and selecting the magnetic nanotubes based on diameter and length. The patent describes positioning the nanotubes attached to magnetic nanoparticles at the desired locations on the wafers by magnetically assisted assembly.
U.S. Pat. No. 6,809,361 to Wolfgang Honlein and Franz Kreupl describes a magnetic memory unit that has two magnetizable electrodes and a nanotube between the electrodes. Information is stored in the memory unit by setting a magnetization direction in one of the magnetizable electrodes by applying an external magnetic field. The magnetic memory functions by spin-polarizing the electrons in the first magnetizable electrode and transporting the electrodes without any change in the spin state through the nanotube into the second magnetizable electrode.